74 ORGANIC FUNCTIONING 



similar to those of the muscle-fibres. Little that is exact is known 

 as to the nature of ciliary movements and much more is known 

 about the muscle-fibre. 



25. ON THE NATURE OF MUSCULAR CONTRACTION 

 A muscle, however it is disposed, always has its tendinous 

 extremities attached to parts that are relatively fixed on the one 

 hand, and '' relatively movable," on the other. Thus the biceps 

 muscle of the human arm " originates " in tendons attached to 

 the humerus and this is the '' relatively fixed " part. The other 

 end of the muscle is " inserted " into the ulna and this is the 

 '' relatively movable " part. In all states the muscle is in a 

 condition of slight tension (or " tonus "), but when it is going 

 to act the tension between its two tendons suddenly increases 

 and forces are applied to the parts into which these tendons are 

 attached. One part (the humerus, in the above example) is 

 relatively fixed, being held so by the tensions of other muscles, 

 but the other part (the ulna) is free to move. We say that con- 

 traction of the biceps flexes the arm — that is, bends the forearm 

 on the upper arm, via the elbow-joint. 



z^a. Structure of a Muscle. The essential parts are 

 muscle-cells, which are short fibres, say from one-half to over 

 an inch in length. Each fibre is bounded by a sheath and all 

 the sheaths adhere to each other and are continuous with the 

 tendons. The muscle-fibres are all colinear with each other. 

 An artery carries blood to the muscle and it breaks up into 

 arterioles and capillaries that ramify between the fibres. Blood 

 thus flows through the capillaries and its liquid part passes through 

 the capillary wall and fills up chinks between the muscle-fibres. 

 A vein carries away spent blood from the muscle. Nerves go and 

 come from the muscle. Lymphatic vessels are also connected 

 with the apparatus. 



256. The Mechanism of a Muscle Contraction. The 

 (motor) nerve that goes to the muscle breaks up into fine fibrils 

 and one fibril goes to each muscle-fibre, where it terminates 

 in a structure called the motor-plate. When a nervous impulse 

 passes along the nerve into the muscle the latter contracts. 

 Nervous impulses are momentary ones, so that the effect of one 

 will be that the muscle momentarily twitches. A single nervous 



